The techniques and materials used in the fabrication of integrated circuits and semiconductors have been applied to a variety of different problems where the very small scale inherent in the devices fabricated with these materials and techniques offer advantages to the end users. Despite the very small scale, the precision fabrication techniques that have enabled the fabrication of complex integrated circuits, such as modern microprocessors, have allowed other very intricate devices that are not integrated circuits to be designed and built using these techniques. One good example of the complexity possible in such devices is International Patent Application No. PCT/US94/10385, International Publication Number WO 95/12894 entitled “Micromachined Mass Spectrometer.” This disclosure describes a micromachined mass spectrometer fabricated on a silicon substrate.
While complicated devices such as the micromachined mass spectrometer have been developed, there still exist drawbacks inherent in building such structures out of silicon materials. To over come these drawbacks, a number of new materials have been investigated for the development of microscale devices. Among these materials are photosensitive materials, which are materials that undergo a physical change upon exposure to energy, for example light or electrons. A good example of a photosensitive material is SU-8, manufactured by the Microchem Corp. of Newton, Mass. SU-8 is a negative, epoxy-type, near-UV photoresist based on EPON SU-8 epoxy resin (from Shell Chemical) that was originally developed, and patented by IBM (U.S. Pat. No. 4,882,245 (1989), the entire contents of which are incorporated herein by this reference). Upon exposure to near UV light, SU-8 cross-links and forms highly stable, polymerized bonds. This allows SU-8 to be formed into highly complex patterns, as a solvent can then be applied to the portion of the material that isn't cross-linked to remove it. Accordingly, by carefully controlling the exposure of the material to near UV light, complex patterns in a single layer of SU-8 are easily formed.
Single layers of SU-8 have been shown to be as thick as 2 mm and aspect ratios of greater than 20 have been demonstrated with standard contact lithography equipment. These results are generally attributed to the low optical absorption of the material in the UV range which only limits the thickness to 2 mm for the 365 nm-wavelength, where the photo-resist is the most sensitive (i.e., for this thickness 100% absorption occurs). These properties have naturally led to significant interest in SU-8 for use in microscale devices. For example, Sotec Microsystems, 11 avenue des Baumettes, 1020 RENENS, Switzerland, has sought to develop and commercially market microscale devices based almost exclusively on SU-8 as a material of construction.
Despite the significant energy that has been focused into using materials such as SU-8 as materials of construction for microscale devices, and the advantages inherent in the high aspect ratios enabled by photosensitive materials such as SU-8, the development of practical devices that fulfill needs in the marketplace built with these materials has not been overwhelming. One drawback that may have hindered the wide adaptation of devices fabricated with such materials is a failure on the part of designers to devise simple methods and techniques for readily using these materials in the fabrication of complex, three-dimensional structures, particularly methods and techniques that lend themselves to high levels of automation, and thus repeatable results. Thus, there exists a need for methods and techniques that allow the development of complex three-dimensional structures utilizing photosensitive materials.